Joon Ahn

Kookmin University · School of Mechanical Engineering

The present paper is aiming at describing the injectant behaviors from two rows of film cooling holes with opposite orientation angles. Boundary layer temperature distributions were measured in the streamwise normal plane to illustrate the interaction between the injectant from the holes in the upstream row and from the downstream row. Detailed adi...

The internal cooling passage of a gas turbine blade can be modeled as a ribbed channel. So far, most studies have considered square ribs. However, the ribs can be rounded due to improper manufacturing or wear during the operation. Round ribs have also been tested expecting that they may enhance the thermal and aerodynamic performance. Hence, we hav...

A novel oxy-fuel burner was devised and integrated into a 50 kW-class furnace-type boiler system. A series of experiments was conducted to verify its feasibility for industrial applications. Additionally, numerical simulations were performed and the results validated against experimental data on the detailed physics inside the conventional-design c...

Ribbed channels are widely used to enhance heat transfer in various heat exchange equipment. However, the heat transfer is locally deteriorated immediately behind the rib due to the flow separation. To overcome this shortcoming, a detached rib array has been proposed recently. In the present study, large eddy simulations (LES) of turbulent flow and...

An immersed-boundary method is proposed for the analysis of conjugate problems of convective heat transfer in conducting solids. Inside the solid body, momentum forcing is applied to set the velocity to zero. A thermal conductivity ratio and a heat capacity ratio, between the solid body and the fluid, are introduced so that the energy equation is r...

Brush seals are extensively used in rotating equipment, such as gas turbines and compressors, providing effective sealing while accommodating radial, axial, and angular movements between components. In this study, the performance of brush seals with and without clearances was predicted through axisymmetric 2D computational fluid dynamic (CFD) simul...

This study aimed to develop a heat recovery system for a 1-MW-class cogeneration system with a gas engine. The system recovers heat from two engine components—jacket water and exhaust gas. A plate-type heat exchanger captures heat from the jacket water, contributing to room heating and hot water supply. Additionally, the exhaust gas heat is harness...

This article provides an overview of current research issues in the areas of heating, cooling, ventilation, sanitation, and indoor environments of buildings and plant facilities by reviewing the papers published in the Korean Journal of Air-Conditioning and Refrigeration Engineering in 2023. The following is a summary of the reviewed papers. (1) Th...

In this study, a series of computational fluid dynamics (CFD) simulations was performed on a small-diameter finned tube heat exchanger equipped with wavy-slit fins. Heat transfer and pressure drop characteristics were analyzed according to changes in three geometric variables related to the slit: height, length, and width. As the slit height increa...

The sealing performance of the labyrinth seal applied to the gas turbine secondary passage system was predicted using Flomaster, a one-dimensional code. The rotation effect was predicted by applying clearance change and swirl models, respectively. It is difficult to obtain data on clearance changes due to rotation, and when applied, the leakage pre...

Industrial boilers cause significant energy wastage that could be mitigated with oxy-fuel combustion versus traditional air combustion. Despite several feasibility studies on oxy-fuel burners, they are widely avoided in industry due to major infrastructural challenges. This study measured the performance and heat transfer characteristics of each co...

The objective of this study is to design a heat-recovery boiler for a 1 MW-class gas engine-based cogeneration system intended to recover heat from engine exhaust gas. To accommodate high-pressure applications, a water tube-type boiler is selected. For ensuring effective water circulation within the boiler, an innovative modular-type boiler based o...

The air conditioner indoor unit comprises a fin-tube heat exchanger, and the non-uniformity of the air flow in the tube bank significantly affects the cooling performance. This problem can be solved by effectively configuring a refrigerant supply circuit; however, this requires information on the air flow distribution for each pipe. In this study,...

Air conditioner (AC) systems are important for maintaining indoor thermal comfort, but discrepancies can exist between setpoints and the actual room temperature. In the present study conducted in a typical Korean office during summer, we identified differences of up to 2.77 °C between the AC setpoint and the average room temperature. This variation...

Computational fluid dynamics (CFD)-based advanced numerical optimization techniques are essential as practical tools used to enhance the performance of various fluid machines and fluid devices for realizing carbon neutrality [...]

Staggered labyrinth seals are used in gas turbines for cooling turbine blades. In this study, numerical analyses were performed to evaluate the discharge coefficients about the shape variables of staggered labyrinth seals. Four shape variables were considered: cavity width (CW), staggered height (GH), staggered width (GW), and staggered position (G...

Non-furnace boilers can improve the efficiency of industrial once-through boilers. However, temperature non-uniformity occurs in the economizer connected vertically to the boiler. Heat transfer performance is degraded by temperature non-uniformity. To solve this problem, a corbel was installed on the side wall of the economizer, and a baffle was in...

Immersed boundary method (IBM) was developed that can solve conjugate heat transfer (CHT) including phase change to simultaneously analyze phase change material (PCM) and forced convection. The CHT was analyzed by determining the effective conductivity to satisfy the continuity of temperature and heat flux at the interface. The PCM was included in...

Herein, 50 articles published over the past 20 years on using large eddy simulation (LES) for the internal cooling passage of a gas turbine, especially the mid-chord ribbed channel, are reviewed for the first time. First, the numerical challenges of performing LES on a ribbed channel and experimental verification are summarized. Next, LES data and...

Film cooling has dramatically contributed to the performance improvement of gas turbines, as it is a very effective cooling technique for gas turbines. Large eddy simulation (LES) began to be used in the study of film cooling 20 years ago, and meaningful results have been found, but it has not yet been intensively reviewed. In this review paper, we...

Fin-tube heat exchangers are used in many industrial fields, and plate-type fin-tube heat exchangers are widely used in industrial boilers and cooling/heating equipment. In this study, heat transfer and pressure drop characteristics were analyzed according to the change in the tube diameter, transverse, and longitudinal tube pitch, and fin spacing...

Heat pumps and refrigeration are key technologies to realize carbon neutrality, and active research is being conducted around the world [...]

A series of numerical simulations were performed to study the discharge coefficient based on the geometric parameters of a stepped labyrinth seal that sealed the secondary flow path of a gas turbine. In contrast with straight-through seals, stepped labyrinth seals introduce additional geometrical parameters related to the steps. In this study, thre...

Large eddy simulations were performed for the conjugate heat transfer in a ribbed channel with a geometry, that mimics the internal cooling passage of a gas turbine, using 566, 100, 10, and 1 as the solid and fluid thermal conductivity ratios (K*) and 30,000, 7000 (turbulent flow), and 1000 (laminar flow) as the Reynolds numbers. A fully coupled si...

Utilizing biological processes for hydrogen production via gasification is a promising alternative method to coal gasification. The present study proposes a dynamic simulation model that uses a one-dimensional heat-transfer analysis method to simulate a biohydrogen production system. The proposed model is based on an existing experimental design se...

The main flow could be unsteady in flow fields of film cooling for several reasons such as flow interactions between the rotor and the stator in the turbine. Understanding the characteristics of the film-cooling flow with an unsteady flow is important in the design of gas turbines. The effects of 36-Hz pulsations in the main flow on the streamwise...

The effects of flow structure on the separation efficiency and pressure drop in a cyclone-type oil separator were investigated via Reynolds-averaged Navier-Stokes (RANS) simulations and large eddy simulations (LES), and the results were compared with experiments. Compared with the RANS simulations, the LES results were more similar to the experimen...

Large eddy simulation (LES) and Reynolds averaged Navier-Stokes simulation (RANS) of leakage flow in straight-through and stepped labyrinth seals were performed in order to compare their performances in sealing the secondary flow passage of the gas turbine based on the respective discharge coefficients. The results indicate a 17.8% higher leakage p...

The effects of pulsations in the main flow on film cooling from a cylindrical hole with a spanwise injection angle (orientation angle) are analyzed using numerical methods. The hole is located on a flat plate with a 35° inclined injection angle, and the compound angle denotes the orientation and inclination angles. The film cooling flow fields for...

A series of large eddy simulations was conducted to analyze conjugate heat transfer characteristics in a ribbed channel. The cross section of the rib is square and the blockage ratio is 0.1. The pitch between the ribs is 10 times the rib height. The Reynolds number of the channel is 30,000. In the simulations, the effect of the thermal resistance o...

This study investigates the performance and the indoor thermal environment of hybrid desiccant cooling driven by CHP under four representative ambient conditions, using TRNSYS. Less regeneration heat was required in order of hot and dry, cool and dry, hot and humid, and cool and humid conditions with 0, 1.5, 4.9, and 6.1 kW, respectively. Cooling c...

A 0.5 MW class oxy-fuel boiler was developed to capture CO2 from exhaust gas. We adopted natural gas as the fuel for industrial boilers and identified characteristics different from those of pulverized coal, which has been studied for power plants. We also examined oxy-fuel combustion without flue gas recirculation (FGR), which is not commonly adop...

Large eddy simulations are performed to analyze the conjugate heat transfer of turbulent flow in a ribbed channel with a heat-conducting solid wall. An immersed boundary method (IBM) is used to determine the effect of heat transfer in the solid region on that in the fluid region in a unitary computational domain. To satisfy the continuity of the he...

The forward expansion hole improves the film cooling effectiveness by reducing the penetration of the coolant jet into the main flow compared to the cylindrical holes. In addition, compound angles improve the film cooling effectiveness by promoting the lateral spreading of the coolant on a wall. Evidently, the combination of a compound angle and sh...

In a field test of a hybrid desiccant cooling system (HDCS) linked to a gas engine cogeneration system (the latter system is hereafter referred to as the combined heat and power (CHP) system), in the cooling operation mode, the exhaust heat remained and the latent heat removal was insufficient. In this study, the performance of an HDCS was simulate...

In a triple-hole system comprising a primary hole and two sister holes, when the sister holes are positioned slightly downstream of the main hole under steady flow conditions, their jets generate an anti-counter-rotating vortex pair. Vortex interactions between the jets increase the effectiveness of adiabatic film cooling. In this study, a series o...

A straight-through labyrinth seal is one of the most popular non-contacting annular seals through which energy dissipation by turbulence viscosity interaction is achieved with a series of teeth and cavities. The geometric parameters of the straight-through labyrinth seal, such as clearance , tooth width, tooth height, cavity width, and tooth inclin...

A large eddy simulation (LES) was performed for film cooling in the gas turbine blade involving spanwise injection angles (orientation angles). For a streamwise coolant injection angle (inclination angle) of 35°, the effects of the orientation angle were compared considering a simple angle of 0° and 30°. Two ratios of the coolant to main flow mass...

The effects of bulk flow pulsations on film cooling in gas turbine blades were investigated by conducting large eddy simulation (LES) and Reynolds-averaged Navier-Stokes simulation (RANS). The film cooling flow fields under 32 Hz pulsation in the mainstream from a cylindrical hole inclined 35° to a flat plate at the average blowing ratio of M = 0.5...

This study investigated the effect of adding two sister holes placed downstream the main hole on film cooling by employing large eddy simulation. Here, film-cooling flow fields from a triple-hole system inclined by 35 • to a flat plate at blowing ratios of M = 0.5 and unity were simulated. Each sister hole supplies a cooling fluid at a flow rate th...

In this study, local measurement and computational fluid dynamics (CFD) were employed to evaluate the thermal comfort in a residential environment where desiccant cooling is performed in an outdoor air condition, which is the typical summer weather in Korea. The desiccant cooling system in the present study has been developed for multi-room control...

The effects of sinusoidal oscillations in the main flow on film cooling in the gas turbine were investigated by Large Eddy Simulation (LES). The film cooling flow fields for the sinusoidal oscillation of 32 Hz in the mainstream from a cylindrical hole inclined by 35° to a flat plate at average blowing ratio of M = 0.5 were numerically simulated. Th...

In this study, the combustion process of a Korean wood pellet was observed through experiments and numerical analysis from 500 °C which is low temperature. Based on the experimental results, the combustion process was classified into gasification, flame burning, and char burning. In every phase of the combustion process, flame shape, mass change, a...

In this study, a coaxial type burner capable of applying air combustion, oxy-fuel combustion, and oxy-fuel oxygen combustion with FGR (flue gas recirculation) was developed and tested for combustion and exhaust gas characteristics by installing it in a 3 MW class flame-tube type boiler. Compared with the results obtained from a 0.5 MW class boiler...

V-shaped ribs were investigated with the objective of enhancing heat transfer at the high-temperature side of a fin-plate type heat exchanger used in a railway vehicle compressor. The high-temperature side supplied with compressed air has a Reynolds number of approximately 4000, which is not sufficiently high to generate strong turbulence. When a 3...

The effects of oil wettability on the performance of a cyclone-type oil separator was studied through in situ experimentation and in a real refrigeration system. Based on previous research, the geometry of the oil separator in the present study was designed with an oil recovery device installed at its bottom to mount it on the actual refrigerator....

A grate-firing boiler was developed for wood pellet fuel, and its combustion and heat transfer characteristics were tested. The first prototype revealed problems with fuel delivery and residence time. To solve these problems, we quadrupled the number of grates and developed the second prototype with a baffle; however, the volume significantly incre...

A prototype of a 230 kW-class wood pellet boiler employing a four-step grate, was manufactured and tested. The flame extended from the grate to the exit of the combustion chamber, and the combustion efficiency was found to be unsatisfactory. In order to resolve this problem, the number of fire grates was increased to 16, and the combustion chamber...

Numerical analysis was performed in order to design the shape of the oil separator for the cyclone-type used in the refrigeration system, which can raise the separation efficiency, and which can reduce the pressure drop. Based on the result of the existing research, the total length of the oil separator, the length of the outlet-port, and the pitch...

It is a essential to minimize production of by-products for economically effective petrochemical process. In order to find key factor to achieve the effective process, 2-dimensional computational fluid dynamics considering a variety of physics such as convective and radiative heat transfer and thermal cracking of ethane are carried out. The reactor...

A simple fabrication technique is described for preparing Ag-doped ZnO nanorods using a rapid hydrothermal technique to create a superamphiphilic surface on SS-400 substrates. The high roughness resulting from Ag-doped ZnO nanorod surfaces is responsible for generating the superamphiphilic property of the SS-400 substrate. The fabricated surface sh...

In a gas engine based cogeneration system, heat may be recovered from two parts: Jacket water and exhaust gas. The heat from the jacket water is often recovered using a plate-type heat exchanger, and is used for room heating and/or hot water supply applications. Depending on the operating conditions of an engine and heat recovery system, there may...

Thermal cracking is commonly modeled as plug flow reaction, neglecting the lateral gradients present. In this paper, 2-dimensional computational fluid dynamics including turbulence model and molecular reaction scheme are carried out. This simulation is solved by means of coupled implicit scheme for stable convergence of solution. The reactor is mod...

This research proposed a novel shape design by integrating the geometry that showed the best performance including fin length, pitch and angle on the high temperature heat exchanger of Stirling engine designed for the prime mover of 1kW cogeneration system for home. First, the numerical simulation was conducted on the new design and existing shape,...

Conjugate heat transfer analysis for an ethylene furnace was carried out based on numerical simulation. Detailed distributions of velocity vectors, chemical species, and temperature inside the furnace are presented and discussed. Von Mises stress and heat flux at the tube surface were also evaluated to elucidate mechanisms regarding failure of the...

In order to improve a refrigeration system’s energy efficiency, the separation efficiency of its oil separator should be improved. To do so, we turned to nature for inspiration. A Namib Desert beetle collects water in fog by using its skin, which combines hydrophilic and hydrophobic surfaces. Inspired by nature’s design, we applied a surface with o...

A series of numerical simulations have been carried out to study the performances of cyclone-type oil separators, which are designed for refrigeration-system compressors. The corresponding working fluid is R22, which is a typical refrigerant, whereby a mineral-oil droplet is supplied (Ed-highlight-My interpretation). The outlet-tube length in relat...

Recent years have witnessed a growing concern over saving energy because of global warming issues and energy price hikes caused by increased oil prices. The need to improve energy efficiency to reduce energy consumption has been raised. Refrigeration systems are also expected to have their energy efficiency improved. A refrigeration system’s the co...

A series of field tests of hybrid desiccant cooling systems was conducted during July and August 2012. The temperature and humidity of supply and return air and power and heat consumption were monitored, and data were transferred in real time through the internet. The performance of the cooling system (i.e., cooling capacity and the coefficient of...

A series of numerical simulation has been carried out to study performance of a cyclone type oil separator, which is designed for the compressor of a refrigeration system. Working fluid is R22, which is a typical refrigerant, and mineral oil droplet is supplied. Pitch of the helix is considered as design parameters to make a compact separator. Depe...

A hybrid desiccant cooling system (HDCS) that uses a heat pump driven by district heating instead of a sensible rotor can provide an increased energy efficiency in summer. In this paper, the summer operation costs and initial costs of both the HDCS and traditional systems are analyzed using annual equal payments, and national benefits are found fro...

A series of field tests on hybrid desiccant cooling systems were conducted in July-August, 2013. The temperature and humidity of the supply and return air, power, and heat consumption were monitored and transferred in real time through the Internet. The performance parameters of the cooling system, namely, cooling capacity and COP (coefficient of p...

Combustion processes of wood pellets in hot cross flows have been studied on the basis of a series of unsteady simulations. Results show that the pellet ignition occurs with a cross flow temperature higher than 600°C. The locus of the ignition point changes depending on the free stream temperature, while combustion of char occurs throughout the pel...

A series of numerical simulation has been carried out to study thermo-hydraulic characteristics of a primary surface type heat exchanger, which is designed for the evaporator and condenser of a geothermal ORC. Working fluid is geothermal water at hot side and R-245fa, which is a refrigerant designed for ORC, at cold side. Amplitude ratio of the cha...

A numerical simulation was conducted for the combustion chamber of a 4-step grate-firing boiler for wood pellet fuel. The flame is extended to the exit of combustion chamber, which is reproduced by present numerical method based on a homogeneous reaction model. Flow field from the simulation shows a strong recirculation flow at the upstream corner...

A grate-firing boiler was developed for wood pellet fuel, and then its combustion characteristics were tested. The flame was stretched to the exit of the combustion chamber, implying insufficient space for complete combustion. As a first step to resolve this problem, a numerical simulation was conducted for the combustion chamber. Turbulent and che...

A series of numerical simulation has been carried out to study thermo-hydraulic characteristics of a primary surface type heat exchanger, which is designed for the evaporator and condenser of a geothermal ORC. Working fluid is geothermal water at hot side and R-245fa, which is a refrigerant designed for ORC, at cold side. Aspect ratio of the channe...

The effects of applying the micro CHP system to the domestic sector in Korea were investigated using annual cooling and heating demand data. Four prime movers, micro gas turbine, PEMFC, gas engine and Stirling engine, were compared for three operational modes. Two way buy-back was assumed for both electricity and heat. The Stirling engine gave the...

Heating and hot water has accounted for 68% of Korea's household energy usage. Boiler makes up the bulk of the heating and hot water production. Hence a highly efficient boiler is needed in order to reduce energy consumption. A condensing boiler that recovers latent heat is known to be highly efficient. However, it is expensive and takes more space...

Field tests of hybrid desiccant cooling systems were conducted from July to August 2011. Data were monitored and transferred in real time over the Internet. The monitored variableswere analyzed to determine the performance characteristics under outdoor conditions. A series of system simulations has been conducted for outdoor conditions of the field...

A fan filter unit (FFU) is a device which supplies clean air from the ceiling in a clean room. With an increase in its size, velocity variation occurs within the exhaust plane and this damage the product quality or productivity. Hence, a guide vane is installed inside the device to enhance the velocity uniformity. Because the vane reduces the flow...

For the optimal operation of steam generation in the HRSG system, the longitudinal pitch distance of the tube banks can be controlled independently for each individual row. To achieve a uniform heat transfer rate for each row of the tube bank, we conducted a full numerical study that includes a variation of the longitudinal pitch for the in-line tu...

As fossil fuel has been depleted, a reasonable use of energy is required. In this regard, the cascade use of thermal energy has been proactively considered in view of available energy. For the purpose, the micro co-generation system for a household based on Stirling engine was suggested as a unit of distributed energy system. A slot-type premixed b...

A finned-tube-type evaporator module has been proposed for a 2 t/h-class water-tube-type industrial boiler with multiple burners. The geometry of the fins was changed at each module to equalize the evaporation. The modules were designed by considering the energy balance at each row rather than by following a conventional bulk design procedure. The...

In this study, a longitudinal pitch effect on in-line tube bank heat transfer has been analyzed numerically. To verify the accuracy of the solver model and boundary conditions, global Nusselt number(Nu) and pressure drop across the 2 row tube bank are compared with the existing experimental correlations under 500 ~ 2,000 Reynolds number(Re) range....

Stirling engine is an external combustion engine, whose efficiency approaches that of Carnot engine with the help of a regenerator. The regenerator is a heat exchanger composed of porous medium, whose performance is dependent on the pore structure. Three types of pore structures are considered in the present study. They are wire screen, random wire...

A modified VIStA (Vortex Inertial Staged Air) burner has been developed and applied to a once-through type boiler. The secondary air is supplied through a swirler instead of nozzles, which stabilizes the flame and reduces carbon monoxide (CO) emissions. However, the modification increases the emission of nitrogen oxides (NOx). To balance emissions...

A finned tube type heat exchange module has been proposed for a multi-burner water tube boiler. Fin density and length increase in streamwise direction to equalize the evaporation for each module, which makes it difficult to apply conventional bulk design procedure. The design program has been improved by updating data for every row of tubes along...